Do you actually want to compose tunes (ie, create new tunes from scratch), or merely play existing tunes?

Normally, the composing is done separately, and the devices simply plays the composition; eg, phone ringtones are downloaded and played by the phone.

However, there are various techniques to automatically compose new "tunes" - based on various patterns, rules, etc. (I think some PC and console games do this).

Of course, once you actually start composing, you get into areas of style, taste, culture, etc, etc,...